Feudum: The Queen’s Army is a solo and co-op/semi-co-op expansion for Feudum, from Odd Bird Games. It pits players against an automated opponent who sets out to kill the game’s behemoth, and then the player or players, scoring veneration points all the while.
While not quite as much of a sandbox, and a little less fulfilling than the core game, particularly with larger player counts, The Queen’s Army does a great job converting that experience to something that is playable for a solo gamer.